An organic light-emitting diode (abbreviated as OLED hereinafter) is a device that converts electric energy to light energy by injecting hole and electron to a light-emitting layer constructed with an organic thin layer. A display device constructed with OLED (abbreviated as OLED display, hereinafter) features the thinness and lightness because no additional light as backlight is required due to the self light-emitting capability. Furthermore, OLED display has other features as wide viewing angle and quick time-response in display characteristics.
FIG. 33 shows an example of a conventional OLED construction and a substantial sectional view of the drawing that explains the display operation. This OLED is constructed with transparent electrode 200 functioning as an anode, hole transporting layer 103, light-emitting layer 102, an electron transporting layer 101, a reflective electrode 300 made of light-reflective metal that functions as a cathode on a transparent substrate 400, layered in this order. Once a DC voltage is applied between the transparent electrode 200 and the reflective electrode 300, the holes injected through the transparent electrode 200 travels in the hole transporting layer 103, the electrons injected through the reflective electrode 300 travels in the electron transporting layer 101. Both holes and electrons reach the light-emitting layer 102 where electron-hole recombination occurs and a light with certain wave length is emitted. A part of the emitted light from the light-emitting layer 102 is observed through the transparent substrate 400 by a viewer 1000. The light that emits in roughly parallel to the boundary surface of the layers or the light that has larger incident angle against the boundary surface than the critical angle between the two layers thereof is propagated in parallel to the boundary surfaces and does not travel to the viewer and therefore they are not effectively used for display lights. The external coupling efficiency (the ratio of amount of the light extracted to the viewer 1000 to the emitted light from the light-emitting layer 102, that, is to say, the ratio of external quantum efficiency to internal quantum efficiency) is generally estimated to be about 20% on the basis of classical ray optics. The large amount of the light generated at the light-emitting layers travels in parallel to the boundary surface of the layers and become a loss in the display system. Therefore, in order to realize low consumption power and bright OLED, it is quite important to reduce the light guiding loss and raise the external coupling efficiency.
The references as Patent 1 and Patent 2 shown below describe OLEDs that have reflection surfaces with the tilted surfaces in order to reduce the guiding loss. For these cases, the description says the light emitted from the light-emitting layers travels in parallel or substantially parallel to the substrate or the layered film, is reflected at the tilted reflective surface and changes the traveling direction, which results into the reduction of light guiding loss and the improvement of the external coupling efficiency.